This invention relates to means for determining the location of aircraft traffic and particularly to the use of standard air traffic control (ATC) transponders to determine the position of aircraft within the terminal control area to a degree of accuracy heretofore not realizable.
There has been disclosed in U.S. Pat. No. 3,870,994 a means for interrogating an ATC transponder located in a small geographical area within a larger field of interest, to the exclusion of other ATC transponders within the field of interest, by transmitting, from a first interrogation station, a message which suppresses all transponders within the field of interest except for transponders which lie inside a narrow beam having its origin at the first station, and thereafter transmitting a standard air traffic control radar beacon system (ATCRBS) message from a second, spaced apart, station. As known to those skilled in the art, the standard ATCRBS message is transmitted by radiating an interrogation message along a narrow beam from the transmitting station. As known to those skilled in the art the interrogation message is adapted to suppress all transponders outside the above mentioned narrow beam from the second station. Thus, a transponder located at the intersection of the beams from the first and second stations will be the only transponder within the field of interest which is not suppressed and which also receives the interrogation message. It will thus be the only transponder within the field of interest which will respond. If the transponder is located on a grounded aircraft, such as an aircraft taxiing at an airport, then the response will provide the aircraft identification. The state of the art is such that this response is easily obtainable.
Geographic addressing by interrogation means as described above is of most utility at close ranges to the interrogation stations due to the relatively long base line between stations as compared to the range of the transponders being interrogated. The aforementioned means is particularly adapted for addressing transponders in aircraft on the ground or in the nearby vicinity of an airport.
The airport terminal control area (TCA), which includes the airport surface and the airspace around the airport out to about a 60 mile range, has long been considered a key problem area in the control of air traffic. The TCA is characterized by converging and increasing air traffic as a function of distance from the airport. Many of the aircraft control problems in the TCA are traceable to the operational limitations imposed by the currently operational surveillance system which is essentially uniform and marginally adequate in its accuracy and resolution capabilities over the entire TCA. Other problems result from actual gaps and voids in coverage, the constraint imposed by presently operational uniformly rotating mechanical antennas which interrogate each aircraft at four second intervals regardless of aircraft position and the general mismatch between real aircraft traffic control needs and the basic performance limitations of existing operation beacon interrogators comprising the present aircraft surveillance system.